Don't think it should make any difference, but just for the record:
- what DBMS are you importing from?
- what version of EA are you using?
One obvious thing (sorry) to check: In your Diagram->Properties (F5), under Connectors, is "Show Relationships" checked?
When I import from SQLServer 2012 using EA-12, underneath each imported table in the browser, I get: columns, PK, FKs, and indexes - it this what you mean by "The tables and constraints are all visible in the Project Browser."?
And just to reinforce what Sunshine says above, when I double-click in the browser on the table so that the properties dialog opens, what I see is entries under "Related"->"Links" that list the associated tables with Connection type "Association", and Stereotype "FK". Its these that show up in my diagrams as the lines between tables, which I think is what you are looking for.
If you cant see any, can you create an FK Association between your tables (i.e. just to prove that it does work in your setup)?